Add template keyword to for Clang

[Jonathan Wakely]

Clang wants this change:

--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -509,7 +509,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

    template<typename _Layout, typename _Mapping>
      concept __mapping_of =
-       is_same_v<typename _Layout::mapping<typename _Mapping::extents_type>,
+       is_same_v<typename _Layout::template mapping<typename
_Mapping::extents_type>,
                 _Mapping>;

    template<typename _Mapping>

to fix:

/home/jwakely/gcc/latest/lib/gcc/x86_64-pc-linux-gnu/16.0.0/../../../../include/c++/16.0.0/mdspan:512:30:
error: use 'template' keyword to treat 'mapping' a
s a dependent template name
 512 |         is_same_v<typename _Layout::mapping<typename
_Mapping::extents_type>,
     |                                     ^


I'll push that on Monday.